A number of processes are known for compacting granular moulding material including using the thrust of pressurized gas for compacting the loosely poured mass of moulding substance. Among these is a process and apparatus described in German AS No. 1,961,234, wherein gas pressure is built up from a pressure container and is then released to act in a thrust-like manner on the moulding mass by actuating a valve which releases the pressurized gas from a hollow space.
Because the volume of compressed gas which is necessary for the compacting must bear a certain ratio to the poured quantity of moulding material, relatively large quantities of compressed air with correspondingly large pressures are necessary for the quantities of moulding material needed in foundry moulds.
Because, on the one hand, a large surface action of the pressure gas thrust onto the moulding mass is needed for good compaction, and on the other hand the valve for controlling the movement of the mass cannot be chosen to be too large, a necessary compromise is to use high gas pressure with a relatively small valve. Then, in order to obtain the compressing action of the gas over a large surface area, a perforated plate for dispersing the pressurized gas over a large area is disposed beneath the valve opening.
However, high pressure, particularly in the range above 20 bar, has many disadvantages with regard to its use in this operation and the arrangement of a perforated plate constitutes an obstacle for effective transfer of the pressure force.